Germanium dioxide, also called germanium oxide and germania, is an inorganic compound, an oxide of germanium. Its chemical formula is GeO2. Its other names are germanic acid, G-15, and ACC10380. It forms as a passivation layer on pure germanium in contact with atmospheric oxygen.

Germanium dioxide is a structural analog of silicon dioxide. Hexagonal GeO2 has structure as β-quartz with coordination number of germanium 4, tetragonal GeO2 has rutile structure of stishovite with coordination number of germanium 6 and amorphous GeO2 is similar to fused silica. It can be prepared both crystalline and amorphous. At very high pressures, germanium dioxide forms unusual octahedral structure. [1][2]

Mixture of silicon dioxide and germanium dioxide ("silica-germania") is used as an optical material for optical fibers and optical waveguides. Controlling the ratio of the elements allows precise control of refractive index. Silica-germania glasses have lower viscosity and higher refractive index than pure silica. Germania replaced titania as the silica dopant for silica fiber, eliminating the need for subsequent heat treatment, which made the fibers brittle.[3]

Germanium dioxide is used as a germanium supplement in some questionable dietary supplements and "miracle cures". High doses of these resulted in several cases of germanium poisonings.

In manufacture of integrated circuits and transistors, germanium dioxide is a rather poor dielectric and is chemically unstable, which is one of the disadvantages of germanium in comparison with silicon.